CD8 + T cells (TCD8+) play a crucial role in eradicating viral infections. TCD8+ recognize MHC class I molecules on the surface of virus-infected cells. The specificity of recognition derives from the interaction of the T cell receptor with class I molecules bearing peptides from viral proteins. Perhaps the best measure of the effectiveness of TCD8+ in viral immunity is the measures that viruses take to avoid TCD8+ recognition. A number of viral proteins have been identified that function to block antigen presentation. The goal of this project is to understand the mechanism that viral proteins use to disable TCD8+ recognition of infected cells. This is important for at least two reasons. First, a number of the viruses that employ this strategy are medically important (human immunodeficiency virus, cytomegalovirus) and it may be possible to thwart viral interference with antigen presentation. Second, it will lead to novel insights into the antigen processing machinery.